This invention relates generally to integrated circuit fabrication, and more particularly to a method for forming a metallic silicide on monocrystalline or polycrystalline silicon.
Metallic silicides have been used as an interconnection material in integrated circuits to overcome the disadvantages of polycrystalline silicon, the primary disadvantage being that the minimum sheet resistivity of polysilicon is about 10 ohms/square. For example, titanium silicide with a sheet resistance less than about 1 ohm/square has been used to improve the performance of large scale integrated circuits employing MOSFETs. See, e.g., J. G. Posa, "Silicides Nudging Out Polysilicon", Electronics, Vol. 54, No. 22, Nov. 3, 1981, pp. 101-102. Metallic silicides thus permit the scaling down of interconnect and gate linewidths which is required to achieve very large scale integration.
One recent development in the use of silicides is a self-aligned process wherein a masking layer is used to expose selected areas on a silicon slice, followed by deposition of a metal layer. The regions overlying the exposed substrate are converted into a metal silicide by annealing, then the non-converted metal is removed by a selective etch. An example of this process is described in U.S. Pat. No. 4,080,719 to Wilting, wherein silicon dioxide is used as a masking layer and platinum silicide is formed on silicon by heating the substrate at 500.degree. C. The non-converted platinum is removed by etching in aqua regia. The self-aligned process eliminates an additional masking step that would normally be required to pattern the interconnects and gate electrodes.
It is desirable that the silicidation process be reproducible, e.g., that the metal silicide formed over the exposed silicon or polysilicon have a predictable, uniform sheet resistance. It is necessary that the exposed silicon be clean and free of oxide in order to provide an intimate contact of metal to silicon. However, even after oxide removal and deglazing, a thin native oxide forms when a silicon slice is exposed to the ambient before metal deposition. This native oxide has been found to be detrimental to the reproducibilty of the silicidation process.
Another problem that occurs in conventional silicidation processes is out-diffusion wherein during the reaction process silicon diffuses through the silicide layer into the meatl layer and reacts with the metal to form metallic silicide outside the original pattern. The linewidth of the original pattern is thus destroyed. In the extreme case, adjacent lines a few microns apart would be shorted by bridging silicide formed by the out-diffused silicon. This has been a major obstacle, for example, to the use of the solid state titanium-silicon reaction in forming titanium silicide on patterned slices.